As is well known in the art, automobile wheels have a tendency to lock, thereby increasing stopping distance, when the vehicle is braked on wet or frozen road surfaces that have low coefficients of friction. Locking of the front wheels also inhibits steering of the vehicle and affects adversely the lateral stability of the vehicle. Anti-lock brake systems generally known in 6he art will maintain slip ratio at the tire road interface at a calibrated value by increasing and decreasing hydraulic brake pressure with a high pulse frequency to prevent the wheels from being locked during braking.
Typically, an anti-lock brake system used in the automobile industry will cause the anti-lock brake system to increase the effective coefficient of friction in direct proportion to slip ratio within a predetermined target slip ratio range. Stability of the vehicle in a lateral direction also changes in accordance with slip ratio, but that relationship is different because lateral stability will deteriorate for any increase in slip ratio. There is no desirable target slip ratio in the control of lateral stability that would correspond to the target slip ratio range for braking forces in the direction of travel of the vehicle.
In known anti-lock brake systems, a master cylinder develops wheel brake pressure in the usual fashion in the absence of wheel slip. If wheel slip is detected, however, the anti-lock brake system will trigger the operation of a solenoid-operated pressure isolation valve that interrupts direct control of the wheel brakes by the master brake cylinder. Hydraulic pressure, during the anti-lock brake control mode, is developed by a hydraulic brake pump, driven by an electric motor, for the purpose of establishing a controlled brake pressure for the wheel brakes. Upon detection of wheel slip, a solenoid-operated hold-and-dump valve located in the brake control circuit relieves pressure at the wheel brakes. An electronic microprocessor controller that responds to speed signals for the vehicle wheels will actuate a pressure isolation valve for each wheel brake and the pressure hold-and-dump valve for each wheel brake to effect a pulsating brake force. The frequency of the pulsations can be controlled by the microprocessor to maintain wheel slip within a desired target slip ratio range.
The operation of the isolation valve and the hold-and-dump valve, in response to the output signals of the microprocessor, has a tendency to develop undesirable pressure pulsations in the anti-lock brake circuits. Further, the hydraulic pressure pump that is used to control brake pressure during the anti-lock brake control mode develops pressure pulsations in the high pressure side of the pump circuit.
Copending patent application Ser. No. 163,658, filed Dec. 7, 1993, describes an elastomeric attenuator assembly that is in fluid communication with the brake pump and with the fluid flow inlet sides of the isolation valves. The attenuator assembly is intended to absorb the energy of the pulse time by compression of the elastomer of the attenuator assembly. Damping orifices are used in cooperation with the elastomer to eliminate pressure pulses, or at least moderate them, thus reducing undesirable noise and vibration in the brake circuits. Patent application Ser. No. 08/163,658 is assigned to the assignee of this invention.